1. Technical Field
The disclosure is related to a frequency generating circuit and a method for adjusting signal frequency thereof, and is particularly related to a voltage-controlled oscillator module, and a method for adjusting signal frequency and a frequency generating system thereof.
2. Background
Referring to FIG. 1, FIG. 1 is a detailed circuit diagram illustrating a conventional voltage-controlled oscillator. A voltage-controlled oscillator (VCO) 110 is implemented based on a ring oscillator 110, which is controlled by a control voltage Vctrl to generate a frequency signal fout. The control voltage Vctrl is output through a unit gain buffer 120 to drive the ring oscillator 110. A unit delay device 112 at each stage of the ring oscillator is an inverter implemented by utilizing a P-type or N-type metal oxide semiconductor transistor. Generally speaking, the unit delay device 112 at each stage of the ring oscillator 110 is very sensitive to a change of the control voltage Vctrl. Therefore, once the control voltage Vctrl slightly changes, a frequency value of the frequency signal fout changes drastically. The characteristic renders the ring oscillator 110 having a larger gain. The gain referred herein indicates a ratio between the frequency value of the frequency signal fout and the voltage value of the control voltage Vctrl, which may be illustrated with a voltage-frequency curve.
Generally speaking, the voltage control oscillator 100 may be applied at different signal transmission interfaces. In correspondence to different signal transmission interfaces, the voltage-controlled oscillator 100 has to provide the frequency signal fout having a corresponding frequency value. The frequency value of the frequency signal fout may be adjusted by adjusting a gain and bias points of the ring oscillator 110. In the conventional art, the gain and bias points of the ring oscillator 110 are adjusted by changing a structure characteristic of the unit gain buffer. However, changing the structure characteristic of the unit gain buffer may render other issues, such as the gain of the ring oscillator being hard to adjust, a larger gain of the ring oscillator rendering a larger noise, and the control voltage Vctrl having a smaller dynamic range and an operating point not set at center of the gain curve. In addition, in some examples, an operational amplifier serving as the unit gain buffer 120 consumes more power to maintain operation with high current density, high bandwidth, and low noise. Therefore, it is necessary to provide a voltage-controlled oscillator whose signal frequency may be adjusted with different transmission interfaces.
Nothing herein should be construed as an admission of knowledge in the prior art of any portion of the present invention. Furthermore, citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention, or that any reference forms a part of the common general knowledge in the art.